1. Field of the Invention
This invention relates to a multiplex transmission system utilizing a so-called CSMA/CD (Carrier Sense Multiple Access/Collision Detection) transmission system.
2. Description of the Related Art
This type of transmission system includes a nondestructive arbitration type CSMA/CD access system used for multiplex transmission for cars as shown in FIG. 1, for example.
In the above system, a network is constructed by connecting a plurality of multiplex nodes such as a meter multiplex node 11, an air-conditioning unit multiplex node 12 for controlling the air conditioner in the car, an air-conditioning switch multiplex node 13 for controlling the operation switches of the air conditioner, a door FL (front left) multiplex node 14 and a door FR (front right) multiplex node 15 to one another via a common multiplex transmission path (multiplex bus) 16 constructed by paired electric lines.
The above multiplex nodes each include a computer for communications and the computers are operated independently of one another. Each computer transmits signals representing vehicle driving information serially to the multiplex bus in units of data frame at a desired timing and in a multiplex mode.
For this reason, contentions of messages may occur on the multiplex bus, and in order to resolve the contention, priorities are assigned to the messages in the conventional system. Each computer effects the priority control according to the transmission procedure based on the assigned priorities without destroying the message. That is, each computer automatically interrupts the transmission of a lower-priority message based on the priority control and continuously effects the transmission of a higher priority message.
Each of the multiplex nodes 11 to 15 returns an ACK signal (acknowledge signal) to the storage location of the frame corresponding to a corresponding one of inherent addresses assigned to the respective nodes when reception of data is normally completed. In an embodiment, the respective nodes are assigned with the inherent addresses 1 to 5, and the meter multiplex node 11, air-conditioning unit multiplex node 12, air-conditioning switch multiplex node 13, door FL multiplex node 14 and door FR multiplex node 15 respectively return ACK signals in response to ACK1 to ACK5, for example.
The above multiplex transmission system has a system construction as is disclosed in U.S. Pat. No. 4,951,281, for example. In the multiplex transmission system, each of the multiplex nodes has an ACK table and registers all of the nodes connected to the multiplex bus into the ACK table in response to the returned ACK signal. Each of the multiplex nodes effects the ACK management operation based on the ACK managing function for newly registering a multiplex node corresponding to the returned ACK signal when the number of returned ACK signals is increased, and canceling the registration of a multiplex node when the multiplex node does not return an ACK signal even if data is repeatedly transmitted thereto a preset number of times.
In the above multiplex transmission system, a problem arises when a multiplex node interrupts and starts to transmit a frame while another multiplex node is retransmitting data. Namely, an ACK signal may be returned from the node to which data is being retransmitted to the interrupt frame. This ACK signal is normal as far as the retransmitted data is concerned, but it causes a situation similar to that where an ACK signal is generated due to noise. In the case of the above multiplex transmission system for cars, the relationship between the transmission nodes and the ACK return nodes is established as shown in the following Table 1, for example.
TABLE 1 ______________________________________ ACK return node Air- Air- con- con- Transmission ditioning ditioning Door Door node Meter unit switch FL FR ______________________________________ (1) Meter .largecircle. .largecircle. .largecircle. .largecircle. .largecircle. (2) Air- .largecircle. .largecircle. .largecircle. X .largecircle. conditioning unit (3) Air- .largecircle. .largecircle. .largecircle. X .largecircle. conditioning unit re- transmission (4) Door FR .largecircle. .largecircle. .largecircle. .largecircle. .largecircle. (5) Air- .largecircle. .largecircle. .largecircle. X .largecircle. conditioning unit re- transmission (6) Air- .largecircle. .largecircle. .largecircle. X .largecircle. conditioning unit re- transmission ______________________________________
In this case, the mark .largecircle. indicates that an ACK signal is returned and the mark x indicates that an ACK signal is not returned.
At the time of transmission by the meter multiplex node 11, the five multiplex nodes 11 to 15 return ACK signals. For this reason, all of the nodes 11 to 15 are registered. Next, at the time of transmission by the air-conditioning unit multiplex node 12, the door FL multiplex node 14 does not return an ACK signal. Therefore, the air-conditioning unit multiplex node 12 effects the first retransmission, but before the retransmission is completed, a frame from the door FR multiplex node 15 having the higher priority than a frame from the air-conditioning unit multiplex node 12 causes an interruption. If, in this case, the door FL multiplex node 14 returns its ACK signal in response to the interruption frame, all nodes register the door FL multiplex node in their ACK tables. As a result, the air-conditioning unit multiplex node 12 stops the retransmission, and since the retransmission is stopped, some nodes cannot receive the data from the air-conditioning unit multiplex node 12. Further, an abnormal node in the network cannot be detected.
That is, in this case, the node which does not detect the abnormal node (door FL multiplex node 14) effects the retransmission without fail. For this reason, in this example, the possibility of collision of messages becomes high and the data transmission amount, that is, the traffic amount is increased so that it may take a long time to transmit the message. Further, if the above condition occurs when the car driving operation is controlled while the car engine control unit and suspension control unit communicate with each other, a certain node detects the abnormal condition and starts to effect the degradation control and other nodes effect the same control operation as in the normal state, and as a result, the performance of the car may be significantly deteriorated.